It is possible to noninvasively acquire information inside an organism by using light having high transmittance to an organism and also having light intensity peak wavelength (described as peak wavelength hereinafter) in a range from a visual area to a near-infrared area. This technique is based on the Lambert-Beer law indicating that an logarithmic value of a detected optical signal is proportional to a product of a light path length by density. Based on this law, for instance, the technique has been developed for measuring relative changes in density of “oxygenated hemoglobin (Hb)” and “deoxygenated Hb” in an organism. Hb is a substance present in an erythrocyte and important for delivering oxygen, and shows different light absorption spectrums when the substance fetches in oxygen and when the substance releases oxygen respectively (refer to FIG. 2). Therefore, by using two types of light having respective different wavelength ranges and measuring changes in transmitted light amplitude of light in each wavelength band (ΔA(λ1), ΔA(λ2)), changes in density of oxygenated Hb and deoxygenated Hb (ΔCoxy, ΔCdeoxy) can be computed from Equation (1) below. In the equation, εoxy and εdeoxy indicate an absorption constant of oxygenated hemoglobin and an absorption constant of deoxygenated hemoglobin, respectively, in each wavelength band.
                                                                                          Δ                  ⁢                                                                          ⁢                                      C                    oxy                                                  =                                                                                                                              -                                                      ɛ                                                          deoxy                              ⁡                                                              (                                                                  λ                                  ⁢                                                                                                                                          ⁢                                  2                                                                )                                                                                                                                    ·                        Δ                                            ⁢                                                                                          ⁢                                              A                                                  (                                                      λ                            ⁢                                                                                                                  ⁢                            1                                                    )                                                                                      +                                                                                            ɛ                                                      deoxy                            ⁢                                                                                                                  ⁢                                                          (                                                              λ                                ⁢                                                                                                                                  ⁢                                1                                                            )                                                                                                      ·                        Δ                                            ⁢                                                                                          ⁢                                              A                                                  (                                                      λ                            ⁢                                                                                                                  ⁢                            2                                                    )                                                                                                      E                                                                                                                          Δ                  ⁢                                                                          ⁢                                      C                    deoxy                                                  =                                                                                                                              ɛ                                                      oxy                            ⁡                                                          (                                                              λ                                ⁢                                                                                                                                  ⁢                                2                                                            )                                                                                                      ·                        Δ                                            ⁢                                                                                          ⁢                                              A                                                  (                                                      λ                            ⁢                                                                                                                  ⁢                            1                                                    )                                                                                      -                                                                                            ɛ                                                      oxy                            ⁢                                                                                                                  ⁢                                                          (                                                              λ                                ⁢                                                                                                                                  ⁢                                1                                                            )                                                                                                      ·                        Δ                                            ⁢                                                                                          ⁢                                              A                                                  (                                                      λ                            ⁢                                                                                                                  ⁢                            2                                                    )                                                                                                      E                                                                                                        E                =                                                                            ɛ                                              deoxy                        ⁡                                                  (                                                      λ                            ⁢                                                                                                                  ⁢                            1                                                    )                                                                                      ·                                          ɛ                                              oxy                        ⁡                                                  (                                                      λ                            ⁢                                                                                                                  ⁢                            2                                                    )                                                                                                      -                                                            ɛ                                              deoxy                        ⁢                                                                                                  ⁢                                                  (                                                      λ                            ⁢                                                                                                                  ⁢                            2                                                    )                                                                                      ·                                          ɛ                                              oxy                        ⁡                                                  (                                                      λ                            ⁢                                                                                                                  ⁢                            1                                                    )                                                                                                                                                        }                            (        1        )            
Changes in the state of oxygen in an organism can be obtained from changes in the densities of oxygenated Hb and deoxygenated Hb, and therefore Hb is used as an index material for oxygen present in a brain. Devices for measuring changes in the density of Hb in an organism are disclosed, for instance, in Patent Documents 1 and 2. Efficacy of these devices is described, for instance, by Atsushi Maki et al. (refer to Non-patent document 1). This document discloses measurement of functions of a human brain by measuring changes in Hb density in the cerebral cortex. More specifically, in association with activation of sensory functions and motor functions of a human, a quantity of blood in cerebral cortex areas controlling the functions locally increases, and therefore activity conditions of a human brain can be assessed by detecting changes in densities of oxygenated Hb or deoxygenated Hb in the areas.    Patent Document 1: Japanese Patent Laid-open No. 9-149903    Patent Document 2: Japanese Patent Laid-open No. 9-98972    Non-patent Document 1: Medical Physics, 22, 1997-2005 (1995)    Non-patent Document 2: Medical Physics, 28(6), 1108-1114 (2001)